1. Field of the Invention
The invention relates to a radio network system for making multihop radio communication by including a plurality of radio node apparatuses (hereinafter, simply referred to as nodes) and to a control node switching method in the radio network system.
2. Description of the Related Art
In recent years, attention has been paid to a short-distance radio communication system called PAN (Personal Area Network). IEEE802.15.4 as an international standard specifies standard specifications in a physical layer and an MAC layer in the short-distance radio communication system.
FIG. 1 shows the structure of a sensor network system in the related art as a radio network system using the short-distance radio communication system. The sensor network system is constituted by a sync node 100 and a plurality of sensor nodes 110 to 131. The sync node 100 collects sensor data from the plurality of sensor nodes 110 to 131 and supplies them to an external network 200. An apparatus on the external network 200, thus, can use the sensor data.
The sensor network system is activated by using the sync node 100 which functions as a primary controller as a start point. A node which wants to associate with the sensor network, for example, the sensor node 120 generates a beacon request (BEACON REQUEST) as a child node and receives a beacon (BEACON) from the parent node which can be accepted.
The sensor node 120 uniquely decides a parent node from the several received beacons. It is now assumed that the parent node is determined to be the sensor node 110 and the sensor node 120 transmits an association request (Association Request) to the sensor node 110. The sensor node 110 as a parent node returns an association response (Association Response) and allocates a network address to the sensor node 120, thereby fetching the sensor node 120 into the sensor network. Network associating procedures according to a relation between the child node and the parent node are sequentially executed in a tree manner by using the sync node 100 as a start point, so that an alignment of the sensor network system including the sync node 100 and the sensor nodes 110 to 131 is accomplished as a whole.
As mentioned above, the sync node in the sensor network system not only functions as an end point of the collected sensor data but also functions as a start point which manages the whole network. A failure of the sync node, therefore, causes an operation stop of the whole network. The operation stop due to a maintenance inspection or a software updating in the sync node likewise denotes a stop of the whole network. Further, when reactivating from the operation stop, an associating procedure of each node is necessary again and working efficiency is restricted. The restriction deteriorates a reliability evaluation of the sensor network and obstructs a spread of its application.
An arrangement for alternating the sync node without stopping the network operation is, therefore, conceivable. With respect to the point, a concept as an alternate PAN coordinator has been presented in the regulation of IEEE802.15.4 as disclosed in “LOCAL AND METROPOLITAN AREA NETWORK—PART15.4B”, IEEE, P802.15.4, REVb/D6 (Non-Patent Document 1) as a part of the standard IEEE802.15.4 by U.S. Society of Electric and Electronics.